Equatorial Coordinates

Right ascension (abbrev. RA; symbol α) is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study In Mathematics and its applications a coordinate system is a system for assigning an n - Tuple of Numbers or scalars to each point In Astronomy and Navigation, the celestial sphere is an imaginary rotating Sphere of "gigantic Radius " The equatorial coordinate system is probably the most widely used Celestial coordinate system, whose equatorial coordinates are Declination (\delta The other coordinate is the declination. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either

Explanation

RA is the celestial equivalent of terrestrial longitude. Longitude (ˈlɒndʒɪˌtjuːd or ˈlɒŋgɪˌtjuːd symbolized by the Greek character Lambda (λ is the east-west Geographic coordinate measurement Both RA and longitude measure an east-west angle along the equator; and both measure from a zero point on the equator. For longitude, the zero point is the Prime Meridian; for RA, the zero point is known as the First Point of Aries, which is the place in the sky where the Sun crosses the celestial equator at the March equinox. The Prime Meridian is the meridian (line of Longitude) at which longitude is defined to be 0° An equinox is the event of the Sun passing over the Earth's equator in its annual cycle An equinox is the event of the Sun passing over the Earth's equator in its annual cycle

RA is measured eastward from the March equinox. An equinox is the event of the Sun passing over the Earth's equator in its annual cycle Any units of angular measure can be used for RA, but it is customarily measured in hours, minutes, and seconds, with 24 hours being equivalent to a full circle. The reason for this choice is that the earth rotates at an approximately constant rate (see sidereal time). Sidereal time is a measure of the position of the Earth in its rotation around its axis or time measured by the apparent Diurnal motion of the Vernal equinox Since a complete circle has 360 degrees, an hour of right ascension is equal to 1/24 of this, or 15 degrees of arc, a single minute of right ascension equal to 15 minutes of arc, and a second of right ascension equal to 15 seconds of arc. This article describes the unit of angle For other meanings see Degree. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. A minute of arc, arcminute, or MOA is a unit of angular measurement, equal to one sixtieth (1/60 of one degree. Sidereal Hour Angle, used in celestial navigation, is similar to RA, but increases westward rather than eastward. It is important not to confuse SHA with the concept of hour angle as it is usually used in astronomy, which is how far west an object is from one's local meridian. In Astronomy, the hour angle is one of the coordinates used in the Equatorial coordinate system for describing the position of a point on the Celestial sphere This article is about the astronomical concept For other uses of the word see Meridian.

RA can be used to determine a star's location and to determine how long it will take for a star to reach a certain point in the sky. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The sky is the part of the Atmosphere or of Outer space visible from the surface of any Astronomical object. For example, if a star with RA = 01:30:00 is at a location's meridian, then a star with RA = 20:00:00 will be in the meridian 18. This article is about the astronomical concept For other uses of the word see Meridian. 5 sidereal hours later.

Since the right ascension (and declination) of stars are constantly changing due to precession, astronomers always specify these with reference to a particular epoch. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either In Astronomy, Precession refers to the movement of the rotational axis of a body such as a planet with respect to Inertial space. In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. The currently used standard epoch is J2000.0, which is January 1, 2000 at 12:00 TT. In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. New Year See also New Year The Ancient Romans began their consular year on January 1st since 153 BC 2000 ( MM) was a Leap year that started on Saturday of the Common Era, in accordance with the Gregorian calendar. Terrestrial Time (TT is the modern astronomical standard for the passage of time on the surface of the Earth (for civil purposes Coordinated Universal The prefix "J" indicates that it is a Julian epoch. Prior to this astronomers used the successive Besselian Epochs B1875. In Astronomy, an epoch is a moment in time used as a reference for the Orbital elements of a Celestial body. 0, B1900. 0 and B1950. 0.

History

The concept of right ascension has been known at least as far back as Hipparchos who measured stars in equatorial coordinates in the 2nd century BC. Hipparchus ( Greek; ca 190 BC &ndash ca 120 BC was a Greek Astronomer, Geographer, and Mathematician of the Hellenistic But Hipparchos and his successors made their star catalogs in ecliptical coordinates, and the use of RA was limited to special cases. Hipparchus ( Greek; ca 190 BC &ndash ca 120 BC was a Greek Astronomer, Geographer, and Mathematician of the Hellenistic A star catalogue, or star catalog, is an Astronomical catalogue that lists Stars In Astronomy, many stars are referred to simply by catalogue The ecliptic coordinate system is a Celestial coordinate system that uses the Ecliptic for its Fundamental plane.

With the invention of the telescope, it became possible for astronomers to observe celestial objects in greater detail, provided that the telescope could be kept pointed at the object for a period of time. A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The easiest way to do that is to use an equatorial mount for the telescope, which allows the telescope to rotate at the same rate as the earth. An equatorial mount is a mount that has one rotational axis parallel to the Earth's axis of rotation Rotation period Earth's rotation period relative to the Sun (its mean solar day is 86400 Seconds of mean solar time As the equatorial mount became widely adopted for observation, the equatorial coordinate system, which includes right ascension, was adopted at the same time for simplicity. The equatorial coordinate system is probably the most widely used Celestial coordinate system, whose equatorial coordinates are Declination (\delta Equatorial mounts could then be accurately pointed at objects with known right ascension and declination by the use of setting circles. In Astronomy, declination (abbrev dec or δ) is one of the two coordinates of the Equatorial coordinate system, the other being either Setting circles are used on Telescopes equipped with an Equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used The first star catalog to use right ascension and declination was John Flamsteed's Historia Coelestis Britannica (1712, 1725). A star catalogue, or star catalog, is an Astronomical catalogue that lists Stars In Astronomy, many stars are referred to simply by catalogue John Flamsteed FRS ( 19 August, 1646 - 31 December, 1719) was an English Astronomer and the first

See also

• Declination, Celestial coordinate system
• geographic coordinates, ecliptic
• Setting circles

External Links

• Right ascension and declination visualized